Electrostatic developing apparatus and method



E. c. RICKER 2,943,950

ELECTROSTATIC DEVELOPING APPARATUS AND METHOD July 5, 1960 Filed May 7. 1953 \MAGE LAYER CAPILLARY CLOUD GENERATOR SOURCE PRESSURE SOU RCE FIG.Z

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4 4 Ti 5 2 0 Z L 5 G .F, 5 4/ m United States Patent ELECTROSTATIC DEVELOPING APPARATUS AND METHOD Eugene C. Ricker, Columbus, Ohio, assignor, by mesne assignments, to Haloid Xerox Inc., a corporation of New York Filed May 7, 1953, Ser. No. 353,520

12 Claims. (Cl. 117- -17.5)

This invention relates in general to the process of electrophotography, generally known as xerography, and in particular to a method and apparatus useful in the development step according to such process.

In the field of xerography it is usual to form an electrostatic latent image on a suitable surface which may for example, be a photoconductive insulating layer on a conductive backing member. This electrostatic latent image can'then be developed by treating it with a finely divided electroscopic material which is electrostatically attracted to the image surface in conformity with the latent image. Since its inception, the xerographic process has been improved to the state in which the photographic results which can be achieved by the process are closely comparable with the results obtainable from conventional photography, and as a result of this rapid improvement in xerographic results there has been a constantly increasing demand for improved techniques and equipment in the development and image forming steps of the process. In order to obtain fine quality results, particularly in the development of continuous tone images, it is necessary to devise means for presenting to the image layer a finely divided electroscopic material in the form of extremely small, uniformly charged particles substantially free from large agglomerates, so that the formation of material deposited on the image surface closely corresponds to the electrostatic charge pattern residing on the surface. It is, of course, obvious that large or non-uniformly charged particles will result in uneven deposition readily apparent to the unaided eye in the form of graininess or improper tone rendition. It is likewise equally obvious that uniform fiow means must be employed to present the electroscopic particles evenly across the entire surface of the image layer so that development in one area will be on a comparable basis with development in all other areas.

In recent improvements in development methods and apparatus, an electroscopic material has been presented to the image bearing surface in the form of air suspension or cloud of the developer particles which suspension has been given electrostatic charging by passing it through an ion zone which may be generated by a corona discharge. One invention of this sort has been disclosed in a co-pending application Serial No. 244,556, now United States Patent 2,725,304. This method is vastly superior to prior developing techniques but it has room for further improvements: first, greater uniformity of charging potential and polarity in the individual electroscopic particles and, second, greater uniformity of particle size through dispersing of or inhibiting formation of comparatively large agglomerates, i.e. aggregates, individual electroscopic particles clumped together in larger units.

It is, therefore, an object of this invention to provide an improved developing means and apparatus for presenting to an electrostatic image layer a uniformly charged suspension or cloud of electroscopic particles, characterized by the improved uniformity of both charg- 2,943,950 Patented July 5, 1960 ing potential and charging polarity and by freedom from agglomerates.

It is another object of the invention to provide an improved xerographic developing method including the step of charging and deagglomerating a suspension of electroscopic particles by means of turbulent flow of such suspension through an elongated zone of restricted fiow.

It is a further object of the invention to provide an improved Xerographic development process comprising generating a cloud of electroscopic particles, passing such cloud through a capillary member wherein it is charged and deagglomerated, and presenting the resulting cloud to an electrostatic latent image bearing surface.

It is still another object of the invention to provide an improved xerographic developing method including the steps of charging and deagglornerating a suspension of electroscopic particles by means of turbulent fiow of such suspension through a capillary tube, whereby a charged and substantially unagglomerated suspension of particles is presented directly to an image bearing surface.

it is an additional object of this invention to provide a new and improved apparatus for generating charging and deagglomerating a cloud of finely divided electroscopic particles.

It is a still further object of the invention to provide a new xerographic developing mechanism comprising a cloud generator, a charging and deagglomerating zone consisting of a constricted zone of turbulent flow, and a development zone comprising an image bearing layer and a conductive member in closely spaced and parallel relationship, wherein the charging and deagglomerating zone conveys the cloud to the developing space between said members.

Still other objects of the invention will in part be obvious and will in part become apparent from the following specification and from the drawings in which:

Figure l is a diagrammatic representation of developing apparatus according to one embodiment of this invention;

Figure 2 is a diagrammatic side View of developing mechanism according to another embodiment of the invention;

Figure 3 is an enlarged fragmentary view of the transition zone of the apparatus disclosed in Figure 2;

Figure 4 is a diagrammatic top View of a cloud generating conditioning system according to still another embodiment of the invention;

Figure 5 is a side elevation of a developing zone with an image bearing member thereon, according to one embodiment of the invention; and

Figure 6 is an end elevation in cross section of the structure shown in Figure 5 with the image bearing member removed therefrom.

In Figure 1 there is shown a simplified mechanism comprising a cloud generator generally designated 10 consisting of a container of chamber 11 having therein a substantial quantity of a finely divided electroscopic material or powder 12. This container 11 is fitted with a removable cover 14 through which is inserted an air duct or tube 15 having a manifold 16 on the lower end thereof. This air duct 15 leads at its external end to a pressure source or air pump 17 designated and adapted to force a gas such as air optionally in pulsating flow through the manifold where plurality of holes or openings 19 in the manifold admit the gas into the body of the container 11. An extremely fine hollow member such as a capillary 20 pierces the cover 14 of container 11 and leads from this container toward an image layer 21 mounted externally on the cloud generator and bearing the electrostatic latent image which is to be developed.

Figure 2 illustrates a modified embodiment of a developing device according to the invention, comprising a pressure source 23 and a powder cloud generator general-1y designated 24 having a capillary or like member 20 leading therefrom to an image member support 25. According to this embodiment there is a suitable pressure source such as an air pump'or a pressure reservoir which may, for example, be a container of a pressurized gas. This pressure source is positioned and adapted to feed air into a tube 15 leading to a member 26 projecting into cloud generator 24.

This cloud generator comprises a suitable box or chamber 27 preferably sealed and therefore relatively dust tight, containing a powder impregnated member 29 optionally in the form of a ribbon. A roll 30 of this ribbon may be suitably mounted within the cloud generator to feed between ribbon guides 31 onto a take up roll 32.

,or drive means such as a motor (not shown) or a chain of gears 33 and 34 operated by drive handle 35.

Optionally the ribbon guides 31 may be mounted not directly against member 25 so that a flow of gas therethrough is directed into the chamber generally but not at the ribbon 20 as it passes through the ribbon guides. Projecting through the cloud generator container wall 27 is the capillary member 20 which terminates directly behind the ribbon guide members 31 in a position to receive the gas, from member 26. Thus the capillary member 20 is adapted to receive the gas from the pressure source, which gas has been carried through the powder impregnated ribbon thereby picking up and suspending electroscopic particles therefrom. Preferably the ribbon 29 should be directly against capillary 20, whereby the particles are drawn from the ribbon by the fast moving escaping gas.

The capillary 20 leads to a transition zone 36 described more fully in connection with Figure 3. In this zone an additional supply of air or gas may, if desired, be drawn in by Venturi efiect or may be added under pressure, and the turbulent gas flow through the capillary may be converted to non-turbulent or laminar flow if desired. In the transition zone, the flow of the powder suspension may be directed normally or perpendicularly into an expansion saucer 37 wherein the flow is caused to spread or expand and is then directed along the upper surface of image member support 25. In use and operation an image layer 21 is placed on top of image member support 25 in a closely spaced parallel relationship so that the flow of cloud suspension passes horizontally between these two members and is thus presented to the electrostatic latent image on the image bearing member.

In Figure 3 there is shown in greater detail the expansion or transition zone generally designated 36 as set forth in Figure 2. This zone comprises a mixing chamber 39 into which feeds the capillary member 20. Somewhat below the end of this capillary member are a plurality of air inlets 40 which, by a Venturi elfect,

allow external air to mix with the flow of the cloud suspension. This mixed suspension is then passed through an outlet 41 from the mixing chamber into the expansion saucer 37 where it impinges normally on a deflecting surface which may, if desired, be an edge or border portion of the image member 21 on image member support 25. In this manner, the mixed suspension is directed outwardly as illustrated by the arrows 42 and is caused to flow substantially straight across the surface of the member 21.

In Figure 4 is illustrated a modification of the device shown in Figure 2. According to this modification a pressure source 23 as described in the previous figures feeds through duct 15 into a wide or fiat jet 26a. A plurality of capillary members 20a, 20b, 20c and 20d are .mounted to project into the cloud generator 24 directly behind the ribbon 29, generally in the manner as described in connection with Figure 2, Where they receive the powder bearing cloud projected from jet 26a. Optionally, the gas fiow from jet 26a may impinge on the ribbon and thus drive the powder free from the ribbon, or alternatively and preferably the capillary 20 or capillaries 20a, 20b, 20c and 20d are positioned virtually flush against the ribbon, whereby the flow of gas into the capillary dislodges and carries with it a portion of the powder material. These capillary members lead preferably in smooth flowing curves from the relatively narrow opening in the positions adjacent to ribbon 29 to a relatively wider spread where they are directed across a considerably wide area along the surface of an image layer or image bearing member 21. The central capillaries 20b and 20c may be curved to increase their length so that a powder cloud generated in the cloud generator 24 is carried through capillary zones of substantially equal length to spread uniformly across the relatively wide area of a large image layer.

In Figures 5 and 6 there is shown a developing zone which may be employed with any or all of the modifications of this machine described in the previous figures. According to this device one or more capillary or like ducts 20 feed to an image support member 25 which preferably has an extremely smooth upper surface extending in horizontal plane. According to the preferred embodiment of this invention, this support member desirably is electrically grounded or may be held at some suitable bias potential. The back and sides of the support member 25 have walls 44 with supporting ledges or shoulders 45 which are adapted to receive and support an image member 21 with the image bearing surface extremely closely spaced in parallel relationship to the smooth upper surface of the support member 25. According to the usual practice of xerography, the image member 21 comprises a conductive backing member 46 for a photoconductive insulating layer on at least one surface thereof, and when such an image bearing memher is used in conjunction with this support, the photoconductive insulating layer 47 is faced toward member 25 and the conductive backing member 45 is preferably electrically grounded.

With reference to the device and mechanisms which have been described certain generalizations can be set forth as illustrating and defining means or methods of obtaining optimum results in the form of xerographic prints or pictures of highest quality. It will be understood, of course, that these generalizations are not to be considered limitations in the operability of the mechanism for the production of a powder cloud or for the charging and deagglomerating thereof, but that they become of extreme importance in determining the photographic quality of the development results.

As illustrative of the first of these generalizations, the diameter of the duct or capillary member 20 should be fine enough so that, under the conditions of operation, a. high degree of turbulence is created within the capillary by gas flow in excess of the critical Reynolds numher. As is well known, turbulence starts above the critical value of about 2,000 and accordingly the duct under the conditions of operation should result in a gas flow therethrough in excess of the Reynolds number of 2,000. Likewise, the diameter of the tube or duct depends on the overall size of the equipment, on the rate of ilow, on the number of ducts, on the size of the electroscopic particles being passed through the capillaries as well as on dependent or independent variables known or as yet unknown. However, for illustration purposes it may be considered that best xerographic results require that this duct diameter be less than 0.050 internal diameter and usually in the range of 0.001 to 0.040. At the present time best results have been obtained with ducts having an internal diameter between about 0.016 and 0.023 or with a plurality of ducts having openings in the order of 0.001 to 0.005 inch with a general and advantageous trend toward more uniform charging and a higher degree of deagglomeration with the capillaries of smaller diameter.

As the second of these generalizations, it has been found that the length of the capillary likewise is dependent on the numerous factors but that for the best practical results certain numerical guides can be presented. Thus, duct lengths of extreme shortness are operable, and adequate charging and deagglomeration can be accomplished using capillaries as short as 0.1" in length, particularly where unusually fine ducts are employed in the order of less than 0.01 inch diameter. it is presently believed and understood that a capillary having a diameter between about 0.016" and 0.020" produces significantly more complete deagglomeration with a duct length of about 15" as compared with a duct length of about 4" but that lengths in this higher range or longer are seldom justified for the reasons that the slight improvement in result is more than defeated by the danger of failure of the equipment. In contradistinction, ducts as short as 0.1 inch operate eflectively with very fine capillaries in the order of 0005 inch diameter or finer. In this connection it is noted that relatively smooth curves in the capillary members appear preferable, if only because of the somewhat lesser danger of clogging of the tubes.

A third and again interdependent variable relates to the gas pressure employed in the generation and the combined charging and deagglornerating of the electroscopic cloud. In the first place, the pressure to be employed necessarily depends somewhat on the nature of the gas which may be used for this purpose, and it will be understood that diflerent pressures may be preferred when air is the suspending gas, than will be used if some other gas such as carbon dioxide or Freon (one of several fluoro-chloro-alkanes) is chosen as the suspending gas. Likewise, the preferred gas pressure or rates of flow will vary substantially in accordance with capillary diameters or lengths. However, as numerical guides it is noted that pressure difierentia'ls through the capillary in the order of lbs. per square inch or higher are sufficient to cause turbulent flow in capillary of the dimensions as described herein. However, substantially more satisfactory xerographic results are achieved with somewhat higher pressures and it is presently preferred to operate with a pressure differential through the capillary-in the order of about25 to about 50 lbs. per square inch with capillaries of the described dimensions. It is obvious, of course, that the rate of gas flow through the tube is practically controlled by the pressure and nature of the gas and by the dimensions of the tube. As an illustrative example, a capillary unit consisting of two tubes or duets with internal diameter of 0.023" and a length of 3%" carried an air flow rate of 9.7 liters per minute of free ah" at a pressure differential of 30 lbs. per square inch. In a similar unit having a duct of 0.016 internal diameter pressures up to 40 lbs. per square inch and flow rates in the order of 2 to 5 liters per minute of free air produced comparable results with however significant decrease in the number of grosssized agglomerates.

Referring to Figures 1, 2 and 4 it is obvious that the source of the pressurized gas may be any suitable source such as, for example, an air pump or like pressure generating member or a suitable present pressurized gas container. Suitable containers of this variety are readily available on the commercial market in the form of gas capsules of carbon dioxide or the like under pressure, or in the form of bombs or the like of gas such as the fluoro-chloro-alkanes. Similarly, a suitable system may employ a pump or generating means optionally, in combination with a pressure chamber, whereby fluctuations in pressure may be avoided or limited. In this connection however, it is observed that certain advantages appear to accrue through the use of at least a slight degree of pulsating pressure as may be most conveniently achieved by the use of a pump or similar mechanism together with an incomplete pulse inhibiting means. In general, however, means or mechanisms for generation or application of constant or varying pressures may be considered suitable in that either or both are fully effective in the production of high quality xerography prints or pictures.

A further generalization applicable to this invention is the selection of suitable electroscopic materials, both from the point of view of composition and from the point of view of particle size. In general, these particles should be grossly smaller than the duct diameters, to the extent that the largest individual particle or agglomerate usually formed in a cloud of such particles prior to deagglomeration should not approach the minimum duct diameter by at least 1 or 2 orders of magnitude. Thus a convenient particle size which results in extremely high quality xerographic prints or pictures contemplates particles having average diameters in the order of 1 micron, and material of this particle size is admirably suited for use in ducts having an internal diameter of as small as 0.0035 or about microns. With particles grossly larger than one micron average diameter there preferably is used a significantly larger capillary. However, for general purposes it may be stated that finer sized particles, in the absence of undue agglomeration will yield a xerographic print or picture more pleasing to the eye even through particles in a size range of about 5 to 10 microns are substantially undetectable as individual particles with the aid of a magnifying glass. The use of finer particle size, particularly sizes significantly below the one micron range, generally require higher pressures, finer diameter capillaries or longer ducts in order to obtain equivalent .deagglomeration. From the point of view of composition of the electroscopic material, highly satisfactory dense black xerographic prints or pictures can be produced with charcoal, carbon blacks, or carbonaceous pigments. Under proper conditions any of the various carbon or lamp black materials may be employed including such material as furnace blacks, channel blacks and the like. In addition, there may be used such material as milled charcoals and similar materials, or, if desired, finely divided materials having added pigment matter. In the latter category are materials such as finely divided resins containing pigments or dyes such as carbonaceous pigments or various coloring pigments, and the like, compositions of this type being preferred where the xerographic print or picture ultimately is .to be made permanent by a fusing process including heat or vapor fusing.

Referring to Figure 2, it is observed that the capillary member .20 is indicated as being electrically grounded. A metal capillary, electrically grounded is a preferred feature particularly where the apparatus is being used for long continued operation and to at least a limited extent is highly desirable for even a short period of operation. When a powder cloud is passing through the capillary container member there is contact between the powder particles and the walls of the capillary, this apparently being the mechanism by which the charging of the particles is brought about, and it is generally observed that rnost powders are charged to negative polarity by grounded metal capillaries. it is, of course, apparent that this charging of the suspension is accompanied 'by an equal and opposite charging of the walls of the capillary with the result that a relatively high charge potential is built up on the capillary member unless conducted away to ground.

In the usual embodiment of the xerographic process, the xerographic print or image layer is provided with a charge of positive polarity and this charge is selectively dissipated by exposure to a light image. In order to produce a xerographic picture or print in which the dark areas of the print correspond to the dark areas of the optical image (this is known as direct reproduction), it is desirable to employ a negatively charged dark colored electroscopic particle, and inasmuch as most pigments or pigmented particles acquire a negative charge by contact with good ohmic conductors such as metals or the like, it is preferred for the capillary to be made of a metallic material or another ohmic type of conductor where negative polarity charging is required. However, under many circumstances, it may be desirable to produce a xerographic print or picture in which dark areas of the optical image are reproduced light areas in the print. (This is known as a reversal reproduction.) To achieve this reversal result several expedients are available including, for example, negative charging of the photoconductive layer or the use of a light colored electroscopic particle to be deposited on a dark background for the ultimate xerographic print or picture. Perhaps a more straightforward approach to this problem is the proper selection of the electroscopic material and the construction material for the capillary member so that the dark colored electroscopic material is positively charged by contact with the capillary wall. By appropriate techniques such a positively charged particle can be made to be repelled by the positively charged areas of the image layer and can be deposited on the uncharged areas. To accomplish this result, a simple expedient consists in constructing the capillary member of a plastic or like material so selected as to charge the electroscopic material positively. Thus, referring to Figure 4, one or more of the capillary members such as capillaries 20a and 20d may be constructed of a metallic members such as capillaries 20b and 20c may be conelectroscopic particles and one or more of the capillary members such as capillaries 20b and 20c may be constructed of a material of negative triboelectric properties, whereby they are adapted to charge the particles of an electroscopic material to positive polarity. Such a device with selective controls to open and close capillary 20a and 20d as one unit and capillary 20b and 200 as the other, can be employed to give direct or reversal development in a single unit.

It will be apparent that reasonable modifications and variations in the invention will be possible and that changes in the apparatus and mechanism may be made wlthin the scope of the invention. It is to be understood that such modification and variations are within the scope and spirit of the invention and that it is to be limited only by the appended claims.

What is claimed is:

1. Apparatus for developing by dusting with powder an electrostatic latent image on an image bearing member, comprising means to create a suspension of powder particles in gas having a pressure of at least about 10 p.s.i., support means for an image bearing member having an electrostatic latent image thereon, a tubular member of restrictive cross section between said means to create a suspension of powder particles in gas and said support means adapted to feed the suspension of powder particles in gas for image development to an electrostatic latent image bearing member on said support means, and means to turbulently flow a gas suspension of particles through the tubular member from said means to create a suspension of powder particles in gas to bring the suspended particles into repeated, triboelectrically charging contact with the walls of said member during passage therethrough and to present to an image bearing member on said support means a suspension of charged particles, said tubular member being so proportioned with respect to a tube having a length of at least about 0.1 inch and a diameter less than about 0.050 inch to produce a flow rate having a Reynolds number in excess of 2,000 when a pressure drop of at least about 10 p.s.i. is maintained therethrough.

2. Apparatus for developing by dusting with powder an electrostatic latent image on an image bearing member, comprising a source of a gas under pressure feeding into a cloud generator adapted to create a suspension of powder particles in said gas having a pressure of at least about 10 p.s.i. feeding out of the cloud generator, support means for an image bearing member having an electrostatic latent image thereon, at least one capillary tube between the cloud generator and said support adapted to feed a suspension of powder particles from the cloud generator for image development to an image bearing member on said support, and means to turbulently flow a gas suspension of particles through the tube from the cloud generator to bring the suspended particles into repeated, triboelectrically charging contact with the walls of said tube during passage therethrough to present a suspension of electrostatically charged powder particles in gas to an image bearing member on said support, said tube being so proportioned with respect to a tube having a length of at least about 0.1 inch and a diameter less than about 0.050 inch to produce a flow rate having a Reynolds number in excess of 2,000 when a pressure drop of at least about 10 p.s.i. is maintained therethrough.

3. Apparatus for developing by dusting with powder an electrostatic latent image of positive polarity on an image bearing member, comprising a source of a gas under pressure feeding into a cloud generator adapted to create a suspension of powder particles in said gas having a pressure of about 10 p.s.i. feeding out of the cloud generator, support means for an image bearing member having an electrostatic latent image thereon, at least one electrically grounded metallic capillary tube between the cloud generator and said support adapted to present a suspension of powder particles in gas for image development to an image bearing member on said support, and means to turbulently flow a gas suspension of particles through the capillary tube from the cloud generator to bring the suspended particles into repeated, triboelectrically charging contact with the walls of said tube during passage therethrough to present a suspension of electrostatically charged powder particles in gas to an image bearing member on said support, said capillary tube being so proportioned with respect to a tube having a length of at least 0.1 inch and a diameter less than about 0.050 inch to produce a flow rate having a Reynolds number in excess of 2,000 when a pressure drop of at least about 10 p.s.i. is maintained therethrough.

4. Apparatus for developing by dusting with powder an electrostatic latent image on an image bearing member, comprising a source of air under pressure feeding into a cloud generator adapted to create a suspension of powder particles in said air having a pressure of at least about 10 p.s.i. feeding out of the cloud generator, support means for an image bearing member having an electrostatic latent image thereon, said support means having a con ductive electrode positioned therein in parallel relation ship to the image bearing member when in position on said support means and separated apart therefrom, at least one capillary tube between the cloud generator and said support adapted to feed the suspension of powder particles in air for image development to the image bearing member when in position on said support means and between the image bearing member and said conductive electrode, and means to turbulently flow an air suspension of particles through the tube from the cloud generator to bring the suspended particles into repeated, triboelectrically charg ing contact with the walls of said tube during passage therethrough to present the suspension of electrostatically charged powder particles in air into the space between the image bearing member and the closely spaced conductive electrode, said tube being so proportioned with respect to a tube having a length of at least about 0.1 inch and a diameter less than about 0.050 inch to produce a flow rate having a Reynolds number in excess of 2,000 when a pressure drop of at least about 10 p.s.i. is maintained therethrough.

5. Apparatus for developing by dusting with powder an electrostatic latent image on an image bearing member, comprising a source of air under pressure of at least about 10 p.s.i. feeding into a cloud generator adapted to create a suspension of powder particles in said air feeding out of the cloud generator, support means for an image bearing member having an electrostatic latent image thereon, said support means having a conductive electrode positioned therein in parallel relationship to the image bearing member when in position on said support means and separated apart therefrom, at least one electrically grounded metallic capillary tube between the cloud generator and said support adapted to feed the suspension of particles in air for image development to an image bearing member on said support, and means to turbulently flow an air suspension of particles through the tube from the cloud generator to bring the suspended particles into repeated, triboelectrically charging contact with the walls of said tube during passage therethrough to present a suspension of electrostatically charged particles into the space between the image bearing member and the closely spaced conductive electrode, said tube being so proportioned with respect to a tube having a length of at least 0.1 inch and a diameter less than about 0.050 inch to produce a flow rate having a Reynolds number in excess of 2,000 when a pressure drop of at least 10 p.s.i. is maintained therethrough.

6. Apparatus for developing by dusting with powder an electrostatic latent image on an image bearing member, comprising means to create a suspension of powder particles in air having a pressure of at least about 10 p.s.i., support means for an image bearing member having an electrostatic latent image thereon, said support means having a conductive electrode positioned therein in parallel relationship to the image bearing member when in position on said support means and separated apart therefrom, a tubular member having a length at least about 0.1 inch and of restrictive and uniform cross section throughout having an internal diameter between about 0.001 and about 0.040 inch between said means to create a suspension of powder particles in air and said support means adapted to feed a suspension of powder particles in air for image development to an lmage bearing member on said support means, and means to turbulently flow an air suspension of particles at a flow rate having a Reynolds number in excess of 2,000 through the tubular member from asid means to create a suspension of powder particles in air to bring the suspended particles into repeated, triboelectrically charging contact wlth the walls of said tubular member during passage therethrough to present a suspension of electrostatically charged particles in air into the space between the image bearing member and the closely spaced conductive electrode.

7. Apparatus for developing by dusting with powder an electrostatic latent image of positive polarity on an image bearing member, comprising a source of air under pressure, a cloud generator, said source of air feeding into said cloud generator and said cloud generator adapted to create a suspension of powder particles in said air having a pressure of at least 10 p.s.i., a supply of pigmented electroscopic powder particles in said generator, support means for an image bearing member having an electrostatic latent image thereon, a transition zone positioned at said support means, at least one electrically grounded metallic capillary tube having a length at least 0.1 inch and having an internal diameter less than 0.050 inch between said cloud generator and said transition zone, and means to turbulently flow an air suspension of particles through the tubular member from the cloud generator at a flow rate having a Reynolds number in excess of 2,000 to bring the suspended particles into repeated, triboelectrically charging contact with the walls of said member during passage therethrough to present a suspension of electrostatically charged particles to said transition zone, said transition zone being adapted to mix additional air with a suspension of particles in air flowing from the tubular member into the transition zone, and

10 said transition zone being adapted to cause the suspension of particles in air flowing out of the transition zone to flow substantially across the surface of the image bearing member when in position on said support means.

8. Apparatus for developing by dusting with powder an electrostatic latent image on an image bearing member, means to create a suspension of powder particles in gas, support means for an image bearing member having an electrostatic latent image thereon, a tubular member of restricted cross section between said means to create a suspension of powder particles in gas and said support means adapted to feed the suspension of powder particles in gas for image development to an image bearing member on said support means, and means to turbulently flow a gas suspension of particles through the tubular member from said means to create the suspension of powder particles in gas to bring the suspended particles into repeated, triboelectrically charging contact with the Walls of said member during passage therethrough to present a suspension of electrostatically charged particles to an image bearing member on said support means, the gas pressure from the source and the cross section of said tubular member being interdependently selected as to create flow through the member in excess of the critical Reynolds number, said tubular member being interdependently selected because of the triboelectric relationship of the material it is composed of as it relates to the material the particles are composed of and depending on the charge making up the electrostatic latent image on the image bearing member.

9. Apparatus for developing by dusting with powder an electrostatic latent image on an image bearing member, comprising a source of a gas under pressure of at least about 10 p.s.i., a cloud generator, said source of gas feeding into said cloud generator and said cloud generator adapted to create a suspension of powder particles in said gas, support means for an image bearing member having an electrostatic latent image thereon, a tubular member having a length of at least 0.1 inch and of restrictive cross section having an internal uniform diameter less than 0.050 inch between said cloud generator and said support adapted to feed a suspension of particles in gas for image development to an image bearing member on said support, and means to turbulently flow a gas suspension of particles through the tubular member from the cloud generator to bring the suspended particles into repeated, triboelectrically charging contact with the walls of said member during passage therethrough to present a suspension of electrostatically charged particles to an image bearing member on said support, the gas pressure from the source and the cross section of said member being interdependently selected as to create flow through the zone in excess of the critical Reynolds number of 2,000, said tubular member being interdependently selected because of the triboelectric relationship of the material it is composed of as it relates to the material the particles are composed of and depending on the charge making up the electrostatic latent image on the image bearing member.

10. Apparatus for developing by dusting with powder an electrostatic latent image of positive polarity on an image bearing member, comprising a source of air under pressure of at least about 10 p.s.i., a cloud generator comprising an enclosed chamber having powder particles disposed therein, said source of air feeding into said cloud generator and said cloud generator adapted to create a suspension of powder particles in said air, support means for an image bearing member, an image bearing member on said support means and having an electrostatic latent image thereon, a transition zone positioned at said support means, at least one electrically grounded metallic capillary tube having a length of at least 0.1 inch and having an internal diameter less than 0.050 inch between said cloud generator and said image bearing member, and means to turbulently flow an air suspension of particles through the tubular member from the cloud generator to bring the suspended particles into repeated, triboelectrically charging contact with the walls of said member during passage therethrough, the air pressure from the source and the cross section of said tube being interdependently selected to create flow through the zone in excess of the critical Reynolds number of 2,000, said tubular member being interdependently selected because of the triboelectric relationship of the material it is composed of as it relates to the material the particles are composed of and depending on the charge making up the electrostatic latent image on the image bearing member.

11. An improved xerographic developing method wherein a charged suspension of powder particles is selectively deposited in image configuration on an electrostatic latent image bearing surface, said method com prising forming a gas suspension of finely divided charcoal powder particles, forcing with a pressure of at least about 10 p.s.i. said gas suspension through a capillary turbulence zone having a length at least about 0.1 inch and having a diameter less than about 0,050 inch at a rate of flow therethrough suflicient to achieve turbulent flow in contradistinction to laminar flow and in excess of the critical Reynolds number of 2,000 whereby the suspended particles are electrostatically charged by contact with the walls of the capillary zone and are deagglomerated, and presenting the suspension of powder particles from said turbulent zone to the electrostatic latent image bearing surface, said particles being sulficiently charged after turbulent flow through said capillary zone to deposit in image configuration on an electrostatic latent image in the absence of additional ion charging.

12. An improved xerographic developing method wherein a charged suspension of powder particles selectively deposit in image configuration on an electrostatic latent image bearing surface, said method comprising feeding with a pressure of at least about 10 psi. a gas suspension of powder particles through a turbulent zone presenting a restrictive area at a rate of flow sufiicient'to achieve turbulence in contradistinction to laminar flow and in excess of the critical Reynolds number of ,000 during passage through said turbulent zone deagglomerating and electrostatically charging the particles, and presenting the particles through said restrictive area to the electrostatic latent image bearing surface, said particles being sufficiently charged after turbulent flow through said turbulent zone to deposit in image configuration on an electrostatic latent image in the absence of additional ion charging.

References Cited in the file of this patent UNITED STATES PATENTS 881,238 Hasbrouck Mar. 10, 1908 911,646 Cook et al. Feb. 9, 1909 1,568,651 Bryson Jan. 5, 1926 2,297,691 Carlson Oct. 6, 1942 2,507,702 Fields May 16, 1950 2,526,178 Weber Oct. 17, 1950 2,551,582 Carlson May 8, 1951 2,590,534 Hampe Mar. 25, 1952 2,618,552 Wise Nov. 18, 1952 2,657,339 Hampe Oct. 27, 1953 2,701,764 Carlson Feb. 8, 1955 2,752,833 Jacob July 3, 1956 2,784,109 Walkup Mar. 5, 1957 FOREIGN PATENTS 605,979 Great Britain Aug. 4, 1948 OTHER REFERENCES A High Voltage DC. Current Generator," R. E. Vollrath, Physical Review, October 15, 1932, vol. 42, pages 298-304. 

1. APPARATUS FOR DEVELOPING BY DUSTING WITH POWDER AN ELECTROSTATIC LATENT IMAGE ON AN IMAGE BEARING MEMBER, COMPRISING MEANS TO CREATE A SUSPENSION OF POWDER PARTICLES IN GAS HAVING A PRESSURE OF AT LEAST ABOUT 10 P.S.I., SUPPORT MEANS FOR AN IMAGE BEARING MEMBER HAVING AN ELECTROSTATIC LATENT IMAGE THEREON, A TUBULAR MEMBER OF RESTRICTIVE CROSS SECTION BETWEEN SAID MEANS TO CREATE A SUSPENSION OF POWDER PARTICLES IN GAS AND SAID SUPPORT MEANS ADAPTED TO FEED THE SUSPENSION OF POWDER PARTICLES IN GAS FOR IMAGE DEVELOPMENT TO AN ELECTROSTATIC LATENT IMAGE BEARING MEMBER ON SAID SUPPORT MEANS, AND MEANS TO TURBULENTLY FLOW A GAS SUSPENSION OF PARTICLES THROUGH THE TUBULAR MEMBER FROM SAID MEANS TO CREATE A SUSPENSION OF POWDER PARTICLES IN GAS TO BRING THE SUSPENDED PARTICLES INTO REPEATED, TRIBOELECTRICALLY CHARGING CONTACT WITH THE WALLS OF SAID MEMBER DURING PASSAGE THERETHROUGH AND TO PRESENT TO AN IMAGE BEARING MEMBER ON SAID SUPPORT MEANS A SUSPENSION OF CHARGED PARTICLES, SAID TUBULAR MEMBER BIENG SO PROPORTIONED WITH RESPECT TO A TUBE HAVING A LENGTH OF AT LEAST ABOUT 0.1 INCH AND A DIAMETER LESS THAN ABOUT 0.050 INCH TO PRODUCE A FLOW RATE HAVING A REYNOLDS NUMBER IN EXCESS OF 2,000 WHEN A PRESSURE DROP OF AT LEAST ABOUT 10 P.S.I. IS MAINTAINED THERETHROUGH. 